Subwoofers are specialized type of loudspeakers which specialize in the low frequency range. A typical frequency response of the subwoofer usually extends between 20 Hz and 50 Hz. However, it doesn’t hurt if the frequency response extends all the way to 200 Hz. This frequency band is still not covered very well by smaller speakers which you can find in common home theater systems.
When purchasing a subwoofer, the upper frequency should be well known so that it ties in with your satellite speakers. The upper frequency of the subwoofer should be close to the lower frequency of your satellite speakers. I would recommend to look at the frequency measurement charts that the manufacturer of both the subwoofer as well as satellite speakers have provided to determine whether these are a good match. As an alternative, you could purchase a home theater in a box set. These sets usually have components which are well matched with one another.
However, they are some specialty items such as wireless rear speakers or wireless subwoofers where you might want to purchase individual components versus going with a home theater in a box set. Purchasing individual components will give you a greater amount of flexibility and also allow you to pick out each component individually. However, this strategy is better suited for people were are more experienced in the field of audio. If you are a novice then you might want to higher the dropout to a subcontractor.
When integrating the subwoofer into your home theater system, you will have to make sure that the subwoofers active in case you AV receiver outputs the subwoofer channel on an RCA output. An RCA output will not offer enough wattage should drive a passive subwoofer and thus the subwoofer itself requires an amplifier. Actually, that means that the subwoofer itself has to be plugged in the power. Therefore, you will need to have power outlets near the subwoofer as well as any active speakers.
Always make sure the subwoofer is well placed and that there are no components which easily vibrate near the subwoofer. Otherwise, you will end up with sound distortion caused by vibrating components. The best way to evaluate the subwoofers is to play material which has natural sound such as earthquakes. Not only will you be able to hear the effect but also feel it.